Any is too much. Us a long straight bit of wood as a gauge and scrape
away at high spots with sandpaper and fill any low ones/
Bugger, oh well :-}

Had a look at this last night, and mine wasn't actually too bad (2mm
at most) - the original wall was done by a plasterer, and it was about
4mm out towards the top of the wall over a 1.5m length